Process for purifying tannic acid



llnited States Patent 2,787,635 PROCESS FOR PURIFYING 'rANNic acreEdward M. Filachione and Fred P. Luvisi, Philadelphia, Pa., assignors tothe United States of America as represented by the Secretary ofAgriculture No Drawing. Application April 8, 1955, Serial No. 500,308

3 Claims. (Cl. 260--473.6) (Granted under Title 35, U. S. Code (1952),sec. 266) A non-exclusive, irrevocable, royalty-free license in theinvention herein described, for all governmental purposes, throughoutthe world, with power to grant sublicenses for such purposes, is herebygranted to the Government of the United States of America.

This application is a continuation-in-part of our copending applicationentitled Purification of Tannins, filed October 13, 1954, Serial Number462,126.

Tannic acid is produced commercially by aqueous extraction of gallnuts.The material thus obtained, however, has a purity of only 80 to 85% andis not sufiiciently pure for many purposes, especially pharmaceuticaluses. While tannic acid is readily soluble in water and many organicsolvents, such solvents usually also dissolve the impurities ordinarilyfound in crude tannic acid and, hence, have little or no utility in thepurification of tannic acid.

The object of this invention is to provide a practical and economicalprocess for the purification of tannic acid.

We have now discovered that methyl isobutyl ketone, hereinafterabbreviated as MIBK, has the unique and unexpected property of being anexcellent solvent for tannic acid while at the same time having verylittle solvent power for the impurities found in crude tannic acid. Italso has the additional advantage that it dissolves very little waterand is itself substantially insoluble in water, thus vastly simplifyingrecovery processes. Dry tannic acid has only limited solubility in MIBK;hence, some water must be present in carrying out the process. Thenecessary water may be added in any convenient way but we prefer todissolve the crude tannic acid in water and then extract the aqueoussolution with MIBK. In order to facilitate extraction of the tannic acidfrom the aqueous solution, the latter should contain a relatively highinitial concentration of the crude acid. This minimizes the volume ofMIBK required for the extraction and also simplifies disposal of theexhausted aqueous raftinate.

The practice of the invention is further illustrated by the followingexperiment:

Twenty-five grams of crude tannic acid of 84.9% purity was dissolved in75 g. of water and the resulting solution was shaken successively withfive 50 ml. portions of MIBK. The combined extracts were evaporated todryness whereby 85% of the original tannic acid was recovered as apowder of 93.2% purity. Repetition of the process yielded a product ofeven higher purity. Continuous counterice current extraction of thecrude aqueous solution with MIBK is more eflicient and is the preferredprocedure for commercial operation of our process.

The process of the invention is especially advantageous when applied tothe aqueous tannic acid solution obtainable by extracting gallnuts withwater, since the necessity of evaporating the aqueous extract to drynessis thereby eliminated. Thus, the most economical, and therefore thepreferred process for producing purified tannic acid consistsessentially of extracting the macerated gallnuts with water in acounter-current fashion to obtain a strong aqueous solution; thenextracting this solution with MIBK, also in a counter-current fashion,to obtain a strong solution of purified acid in the organic solvent; andfinally, evaporating the solvent to recover both the purified tannicacid and the MIBK.

While the process of our invention is highly satisfactory for thepurification of tannic acid, it is of no value for the purification ofthe tannins of canaigre, quebracho, wattle, myrobalans, divi-divi,chestnut, mangrove, valonia or cutch, since these tannins have verylimited solubility in MIBK. It can be used with ganibier tannins but isrelatively inefi'icient because of poor recovery of tannins. The onlyknown tannin for which the process has practical utility is tannic acid.

Sometimes, particularly when the purified tannic acid is to be usedimmediately as an aqueous solution, in recovering the acid from the MIBKsolution it is advantageous to add sufiicient water to leave aconcentrated aqueous solution of tannic acid after distillation of theMIBK. By this procedure the complete recovery of the MIBK is facilitatedand the mechanical handling of the product is facilitated. Also, thedanger of heat damage to the product during mized.

We claim:

1. The process for producing purified tannic acid comprising contactingan aqueous solution of the acid with methyl isobutyl ketone, whereby thetannic acid is dissolved in the ketone, and recovering the tannic acidfrom the resulting ketone solution.

2. The process for producing purified tannic acid from gallnutscomprising macerating the gallnuts, contacting the macerated materialwith water, thus to dissolve the crude tannic acid, contacting the thusobtained aqueous tannic acid solution with methyl isobutyl ketone, thusto dissolve the acid in the ketone, and recovering the acid from theketone solution.

3. The process of claim 2 wherein the step of recovering the tannic acidfrom the methyl isobutyl ketone solution comprises adding an amount ofwater more than sufiicient to form the ketone-water azeotrope and thendistilling the said azeotrope, thus recovering the tannic acid as adistillation residue consisting essentially of an aqueous solution oftannic acid.

References Cited in the file of this patent UNITED STATES PATENTS2,694,725 Filachione et al. Nov. 16, 1954 distillation of the MIBK ismini-

1. THE PROCESS FOR PRODUCING PURIFIED TANNIC ACID COMPRISING CONTACTING AN AQUEOUS SOLUTION OF THE ACID WITH METHYL ISOBUTYL KETONE, WHEREBY THE TANNIC ACID IS DISSOLVED IN THE KETONE, AND RECOVERING THE TANNIC ACID FROM THE RESULTING KETONE SOLUTION. 